Ejector-expansion refrigeration cycles are known to involve the replacement of a conventional expansion valve with a work-producing jet ejector to reduce the enthalpy of the refrigerant entering the evaporator and provide work to assist in the operation of the compressor. An example is disclosed in U.S. Pat. No. 3,277,660. In the basic ejector-expansion refrigeration cycle the high-pressure liquid refrigerant leaving the condenser is utilized as the ejector motive nozzle fluid for partially compressing the saturated vapor leaving the evaporator.
A liquid-vapor mixture exits from the ejector at a pressure between the evaporator pressure and the compressor discharge pressure. The liquid portion of this flow is returned to the evaporator while the vapor portion enters the compressor suction. In essence the result is a two-stage refrigeration system wherein the work which would otherwise be lost in the high-stage expansion process provides the work input for the low stage.
Controls for the ejector-expansion refrigeration cycle are described in U.S. Pat. Nos. 3,670,519 and 3,701,264. In the first of these, it is proposed that flow through the ejector motive nozzle be controlled by mixing vapor from the compressor discharge with the liquid entering the motive nozzle. The procedure is intended to reduce flow by reducing density of the inlet fluid. In the second, it is proposed that the ejector motive nozzle be fitted with a spindle to reduce nozzle cross-sectional area while maintaining a smooth area variation. This procedure is intended to reduce flow by reducing cross-sectional area. Both these procedures avoid throttling the liquid refrigerant before it enters the motive nozzle.
In recent experimental programs with ejector-expansion refrigeration cycles it has been noted that ejector performance is impaired by metastable conditions of the refrigerant in the ejector nozzle. In the rapid expansion of the gas-free liquid refrigerant flashing from the ejector nozzle, boiling is delayed by lack of nucleation sites. Surface tension results in pressures above saturation within the tiny bubbles that result from random molecular motions and as a consequence these bubbles do not grow. The only available nucleation sites are those provided by crevices in the wall of the ejector tube and hence the flow consists of an annulus of vapor surrounding a core of metastable liquid refrigerant. The refrigerant exiting from the nozzle is therefore not in thermodynamic equilibrium and its enthalpy decrease and kinetic energy increase are considerably less than they would be if the refrigerant were in thermodynamic equilibrium. For example, if saturated liquid R-12 at 140 psia is expanded to 40 psia in an isentropic equilibrium process the nozzle outlet velocity is approximately 280 feet per second. On the other hand if the R-12 refrigerant expands isentropically as a metastable liquid which is not in a state of thermodynamic equilibrium the nozzle outlet velocity is only about 100 feet per second. Since kinetic energy is proportional to the square of velocity the kinetic energy in the non-equilibrium condition is only 13 percent of that in the equilibrium condition.
A principal object of the present invention is to reduce the metastability of the refrigerant flow in an ejector nozzle to approximate as closely as possible an equilibrium condition so that the exiting refrigerant achieves maximum nozzle velocity.